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Misuses of MTBF
MTBF is often misused and can be misleading. It is calculated as the average time between failures of a system, but does not represent the actual duration of failure-free periods. A better metric is reliability (R(t)), which shows the probability that a system will operate at a given time. Additionally, the document notes that MTBF is intended for repairable systems, while MTTF is a more accurate term for non-repairable systems, as it is calculated the same way as MTBF under certain assumptions about repair times and part lifetime distributions.
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Misuses of MTBF
- 1. Misuses of MTBF Fred Schenkelberg Reliability Consultant fms@fmsreliability.com www.fmsreliability.com (408) 710-8248
- 2. Outline • MTBF – calculation • MTBF – a very poor four letter acronym • History of Use • It’s Misleading • A better metric • Actually, we’ve been talking about MTTF
- 3.
- 4. Mean (M) • Themean in MTBF • What does it mean to you? (no pun intended!) • Average?
- 5. Start 1000 units, MTBF = 100 368 still alive at 101 hours
- 6. Note the exponentialdecay
- 7. Other Issues • Time– just because it is hours… • Between – not the duration of the failure free period! • Failure – use the customer definition
- 8. History of Use • Remember Slide Rule Mechanical Adding Machines
- 9. History of Use • Early Parts Count based on adding failure rates of components (50’s and early 60’s)
- 12. Use Reliability • R(t)is the probability that a random unit drawn from the population will still be operating by t hours • R(t) is the fraction of all units in the population that will survive by t hours Applied Reliability, 2nd Ed., pg 29
- 13. Use better models/distributions • Weibull • Type I Gumbel • Exponential • Log Normal • Etc.
- 14. Actually… • MTBF isor should be used for repairable systems • MTTF is what I’ve been talking about • MTTF is calculated the same as MTBF when we assume – negligible repair time – Interarrival times as from an independent sample of nonrepairable parts – Expontential distribution for lifetime of parts • See Chap 10, Applied Reliability for more info